EFFECTS OF UNILATERAL STRENGTH TRAINING AND DETRAINING ON BONE-MINERAL DENSITY AND CONTENT IN YOUNG-WOMEN - A STUDY OF MECHANICAL LOADING AND DELOADING ON HUMAN BONES

This study assessed the effect of unilateral strength training at 80% one repetition maximum and of detraining on bone mineral density (BMD, g/cm(-2)) and bone mineral content (Sigma BMC, g) in young women. Twe lve female physiotherapy students trained their left limb by leg press an average of four times per week for 1 year followed by 3 months of detraining. Twelve students served as controls. Repeated bone measurem ents were performed by dual energy X-ray absorptiometry of the lumbar spine, femoral neck, distal femur, patella, proximal tibia, and calcan eus. The training increased the muscle strength of the trained limb, a nd the BMD of the same limb showed a nonsignificant but systematic inc rease in distal femur, patella, and proximal tibia, and in Sigma BMC o f the five measured limb sites (considered an index of the total osteo genic effectiveness of the training). Simultaneously, the muscle stren gth increased in the untrained limb as an evidence of cross-training e ffect. A corresponding small but systematic increase was also seen in BMD of this limb as well as in C BMC, After the cessation of training, leg extension strength was retained but BMD and Sigma BMC of the trai ned and untrained limbs declined towards baseline values in 3 months. The BMD and Sigma BMC values in the control group showed an increasing tendency during the follow-up but the changes were less than 1%. The differences of the changes in BMD and Sigma BMC between the left and r ight limb in the control group, as well as between the same limb in th e training and control groups were nonsignificant. The findings of thi s study indicate that unidirectional strength training, intensive enou gh to induce substantial strength gain, is not an effective stimulus t o increase BMD and BMC in young, physically active women. The unilater al training model turned out to be feasible in these subjects, produci ng a definite cross-training effect in muscle strength and a trend of similar effect in BMD. Further development of the unilateral training model, and studies to test if training produces adaptation in nonloade d bones (i.e., a cross-training effect), are also warranted.